1. Field of the Invention
The present invention relates to a circulating fluid-bed combustion apparatus, which effectively combusts solid fuel such as coal.
2. Description of the Related Arts
Japanese Examined Patent Publication No. 28046/82 discloses a circulating fluid-bed combustion apparatus, which effectively combusts solid fuels by limiting the discharge of nitrogen oxides and sulfur oxides to a low level. In this combustion apparatus, a superficial velocity of fluidized gases in a fluid-bed combustion chamber is a terminal velocity of fluidized particles or more. The fluidized particles accompanying the gases are separated by a separator. The separated particles are returned to the combustion chamber through a circulation circuit. There are two features in this combustion apparatus. A first feature is that the particle residence time is long due to the circulation of the particles. A second feature is that the gases are strongly mixed with the particles by differences of the fluidizing velocities of the gases and particles.
This circulating fluid-bed combustion apparatus is schematically shown in FIG. 13. This apparatus comprises a fluid-bed combustion chamber 1, a separator 2, a particle recycling conduit 3, a heat exchange portion 4 comprising a boiler heat transfer surface and the like. A primary combustion air inlet 5 is positioned at the bottom of the combustion chamber 1. A secondary combustion air inlet 6 is positioned in an intermediate portion of the combustion chamber 1. A cooling surface 7 for recovering combustion heat is positioned in an upper space of the combustion chamber 1.
Pulverized solid fuels such as coal of from about 0.5 mm to 10 mm in particle size and a desulfurizing agent such as lime are charged into a lower portion of the fluid-bed combustion chamber. The solid fuels are fluidized by the primary combustion air blown from the primary combustion air inlet 5 and mixed with particles in the furnace. Since the solid fuels are rapidly mixed with the primary combustion air, the solid fuels are well ignited.
The combustion air is supplied in two stages and two-stage combustion is carried out. Flame under the secondary combution air inlet 6 is reducing flame having an air ratio of 1 or less, and flame above the secondary combustion air inlet 6 is oxidizing flame having an air ratio of 1 or more.
A low-temperature combustion is possible since the homogeneities of temperatures inside the combustion chamber 1 are attained by actively mixing the gases with the particles and a great amount of circulating particles possess a sufficient heat capacity. The temperature inside the combustion chamber 1 is kept at about 850.degree. C. by the cooling surface 7 inside the combustion chamber 1.
Since the particles inside the combustion chamber have a distribution of certain particle sizes, coarse particles incapable of reaching a terminal velocity of the gases are also mixed with gases. The coarse particles blown upward together with fine particles form a fluidized bed. The coarse particles fall toward the bottom of the combustion chamber as the gases and particles move upwards. In this way, the particles circulate inside the combustion chamber. A suspension concentration of the particles in the combustion chamber is high in the lower portion thereof and low in the upper portion thereof due to the circulation of the particles inside the combustion chamber.
The amount of heat transfer on the cooling surface 7 inside the combustion chamber is varied by the suspension concentration of the particles. The amount of the particles blown upward from the bottom is regulated by varying the proportion of the primary and secondary combustion air. The amount of heat transfer on the cooling surface is regulated so that the combustion temperature can be a predetermined temperature.
Fuel particles become finer as combustion proceeds. When the fine particles reach the terminal velocity of the gases, the fine particles together with the gases are discharged out of the combustion chamber 1, and the particles are caught by the separator 2. The particles caught are returned again to the combustion chamber 1 through the recycling conduit 3. A combustible loss of the fuel particles is minimized by repeatedly carrying out the recycling of the gases and particles.
An amount of discharge of nitrogen oxides generated in a combustion process is limited to a low level due to the following reasons:
Firstly, the temperature inside the combustion chamber is low and homogeneous;
Secondly, the gases and particles are combusted in two stages; and
Thirdly, the fuel particles not yet combusted as reducing agents distribute in all the space of the combustion chamber.
On the other hand, since the desulfurizing agents such as lime stone and the like distribute in all the space of the combustion chamber and the desulfurizing agents are retained in the combustion chamber by the recycle of the particles for a long duration of time, an effective desulfurizing reaction is carried out.
Since the prior art circulating fluid-bed combustion apparatus has such a structure as described above, the following problems are pointed out:
(a) It is necessary to pass the high-temperature combustion gases of about 850.degree. C. through the separator 2 immediately after the gases have been discharged out of the fluid-bed combustion chamber 1. A centrifugal separation type hot cyclone, which is usually formed of refractory due to conditions of its use, is used for the separator 2. Due to a great amount of high-temperature gases to be processed, a draft loss in the hot cyclone is large, which necessitates an auxiliary driving force. Moreover, the size of the hot cyclone is substantially equal to that of the fluid-bed combustion chamber. The great size of the hot cyclone requires a great space necessary for the circulating fluid-bed combustion apparatus. Further, due to the refractory used for the hot cyclone, much time is required for starting up the hot cyclone and maintenance labour is increased. PA1 (b) The duration of time in a process from the hot cyclone 2 to the fluid-bed combustion chamber 1 through the recycling conduit 3 requires 10 to 20 sec., which is regarded as a long duration of time. Since this process is repeatedly carried out 10 to 100 times until particulate solid fuel particles are burnt out, a very long duration of time as a whole is required. Accordingly, the time constant of combustion is very large, which leads to a bad following-up during the load fluctuation. PA1 (c) The distribution of the particle suspension concentration and the temperature inside the combustion chamber are regulated by changing the proportion of the distribution of the amount of the primary and secondary combustion air. The effect of denitration by means of the two-stage combustion is also affected by this proportion of the distribution of the primary and secondary combustion air. Accordingly, when an attempt is made to satisfy the conditions of both the temperature inside the combustion chamber and the denitration ability, the range of application of the prior art method is limited.
Relative to the above-described various problems, there is a combustion apparatus disclosed in PCT/ET No. 87/00729. An example of this combustion apparatus is shown in FIG. 14. A labyrinth separator 8, which is a non-centrifugal mechanical separator, is arranged in an upper space of a fluid-bed combustion chamber. A heat exchange portion 4 is arranged on the downstream side of the labyrinth separator 8. In this example, the particles, which were passed through the hot cyclone in the prior apparatus of FIG. 13, are instead circulated in the combustion chamber 1 by means of the labyrinth separator 8. Therefore, the problems due to the hot cyclone are solved. However, since the distribution of the suspension concentration of the particles is varied by the proportion of the distribution of the amount of the primary and the secondary combustion air as in the circulating fluid-bed combustion apparatus with the cyclone, the problem such that the range of operation is limited cannot be solved.